In direct injection spark ignition engines, the engine operates with stratified air/fuel operation in which the combustion chamber contains stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures.
The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during the intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to conventional three-way catalytic converters to reduce CO, HC, and NOx. When operating at air/fuel mixtures lean of stoichiometry, a NOx trap or catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx.
The stratified mode of operation is typically utilized when the engine is operating in light to medium loads. The homogeneous mode of operation is typically used from medium to heavy load operating conditions. In certain conditions, it is necessary to transition from one engine mode of operation to the other. During these mode transitions, it is desired to deliver the requested engine output torque to provide good drive feel. However, in some circumstances, the range of acceptable lean air/fuel ratios of stratified operation do not overlap with the acceptable air/fuel ratios of homogeneous operation. Therefore, during the mode transition, a torque shock occurs because of the step change in engine air/fuel ratio.
One method of preventing the engine torque disturbance during mode transition is to change the injection mode one cylinder at a time according to the required amount of fuel to be injected. This reduces a large torque disturbance to several smaller torque disturbances. Such a method is described in U.S. Pat. No. 5,170,759.
The inventors herein have recognized a disadvantage with the above approach. Even though the large torque jump during mode transition is avoided, there are still several smaller torque jumps experienced. In other words, a single, large torque disturbance is substituted with multiple smaller torque disturbances which are still noticeable by the vehicle driver.